Perpendicular magnetic anisotropy BCC multilayers

ABSTRACT

A magnetic material includes a cobalt layer between opposing iron layers. The iron layers include iron and are body-centered cubic (BCC), the cobalt layer comprises cobalt and is BCC or amorphous, and the magnetic material has a perpendicular magnetic anisotropy (PMA).

PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/744,137, filed on Jun. 19, 2015, entitled“PERPENDICULAR MAGNETIC ANISOTROPY BCC MULTILAYERS,” which claimspriority from U.S. patent application Ser. No. 14/669,337, filed on Mar.26, 2015, entitled “PERPENDICULAR MAGNETIC ANISOTROPY BCC MULTILAYERS,”each application is incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure generally relates to magnetic materials, and morespecifically to magnetic multilayer materials.

A spin torque magnetic random access memory (MRAM) is a type of solidstate, non-volatile memory that uses tunneling magnetoresistance (TMR orMR) to store information. MRAM includes an electrically connected arrayof magnetoresistive memory elements, referred to as magnetic tunneljunctions (MTJs). Each MTJ includes a free layer and fixed/referencelayer that each include a magnetic material layer. The free andfixed/reference layers are separated by a non-magnetic insulating tunnelbarrier. The free layer and the reference layer are magneticallyde-coupled by the tunnel barrier. The free layer has a variablemagnetization direction, and the reference layer has an invariablemagnetization direction.

An MTJ stores information by switching the magnetization state of thefree layer. When the free layer's magnetization direction is parallel tothe reference layer's magnetization direction, the MTJ is in a lowresistance state. Conversely, when the free layer's magnetizationdirection is antiparallel to the reference layer's magnetizationdirection, the MTJ is in a high resistance state. The difference inresistance of the MTJ may be used to indicate a logical ‘1’ or ‘0’,thereby storing a bit of information. The TMR of an MTJ determines thedifference in resistance between the high and low resistance states. Arelatively high difference between the high and low resistance statesfacilitates read operations in the MRAM.

The magnetization direction of the free layer may be changed by a spintorque switched (STT) write method, in which a write current is appliedin a direction perpendicular to the film plane of the magnetic filmsforming the MTJ. The write current has a tunneling magnetoresistiveeffect to change (or reverse) the free layer's magnetization direction.During STT magnetization reversal, the write current for magnetizationreversal is determined by the current density. As the surface area ofthe the MTJ becomes smaller, the write current for reversing the freelayer's magnetization becomes smaller. Therefore, if writing isperformed with fixed current density, the necessary write currentbecomes smaller as the MTJ size becomes smaller.

Compared to MTJs with in-plane magnetic anisotropy, layers withperpendicular magnetic anisotropy (PMA) can lower the necessary writecurrent density. Thus, PMA lowers the total write current used.

SUMMARY

In one embodiment of the present disclosure, a magnetic materialincludes a cobalt layer between opposing iron layers. The iron layerscomprise iron and are body-centered cubic (BCC), the cobalt layercomprises cobalt and is BCC or amorphous, and the magnetic material hasa perpendicular magnetic anisotropy (PMA).

In another embodiment, a magnetic material includes alternating layersof an iron layer and a cobalt layer. The iron layer comprises iron andis BCC, the cobalt layer comprises cobalt and is BCC or amorphous, andthe iron and cobalt layers each have thickness of about 2 to about 10angstroms (Å).

Yet, in another embodiment, a method of making a magnetic materialincludes forming a cobalt layer on a first iron layer and forming asecond iron layer on the cobalt layer. The first and second iron layerscomprise iron and are BCC, the cobalt layer comprises cobalt and is BCCor amorphous, and the magnetic material has a PMA.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1A is a cross-sectional view of a multilayer magnetic materialaccording to an exemplary embodiment;

FIG. 1B is a cross-sectional view the multilayer magnetic material ofFIG. 1A with a magnesium oxide tunnel barrier layer on a first surfaceaccording to an exemplary embodiment;

FIG. 1C is a cross-sectional view of the multilayer magnetic material ofFIG. 1A with a tunnel barrier layer on a second surface according to anexemplary embodiment;

FIG. 1D is a cross-sectional view of the multilayer magnetic material ofFIG. 1A with a tunnel barrier layer on two surfaces according to anexemplary embodiment;

FIG. 2A is a cross-sectional view of the multilayer magnetic material ofFIG. 1B with a dusting layer disposed between the tunnel barrier layerand the multilayer material according to an exemplary embodiment;

FIG. 2B is a cross-sectional view of the multilayer magnetic material ofFIG. 1C with a dusting layer disposed between the tunnel barrier layerand the multilayer material according to an exemplary embodiment;

FIG. 3A is a cross-sectional view of a tunnel junction including themultilayer magnetic material as a free layer and as a reference layeraccording to an exemplary embodiment;

FIG. 3B is a cross-sectional view of a tunnel junction including themultilayer magnetic material as a free layer and as a reference layer,where the free layer is thinner than the reference layer according to anexemplary embodiment; and

FIG. 3C is a cross-sectional view of the tunnel junction of FIG. 3Bincluding a spacer layer and a magnetic layer according to an exemplaryembodiment.

DETAILED DESCRIPTION

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

As used herein, the articles “a” and “an” preceding an element orcomponent are intended to be nonrestrictive regarding the number ofinstances (i.e. occurrences) of the element or component. Therefore, “a”or “an” should be read to include one or at least one, and the singularword form of the element or component also includes the plural unlessthe number is obviously meant to be singular.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

As used herein, the term “about” modifying the quantity of aningredient, component, or reactant of the invention employed refers tovariation in the numerical quantity that can occur, for example, throughtypical measuring and liquid handling procedures used for makingconcentrates or solutions. Furthermore, variation can occur frominadvertent error in measuring procedures, differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods, and the like. In one aspect, theterm “about” means within 10% of the reported numerical value. Inanother aspect, the term “about” means within 5% of the reportednumerical value. Yet, in another aspect, the term “about” means within10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value.

As used herein, the terms “atomic percent,” “atomic %” and “at. %” meanthe percentage of atoms of a pure substance divided by the total numberof atoms of a compound or composition, multiplied by 100.

As used herein, the terms “body-centered cubic” and “BCC” mean the cubiccrystal lattice has one central lattice point and eight corner latticepoints in each unit cell. As used herein, the terms “face-centeredcubic” and “FCC” mean the cubic crystal lattice has lattice points onthe central faces of the cube and eight corner lattice points in eachunit cell. The crystal lattice structure is determined by transmissionelectron microscopy (TEM) to analyze the electron diffraction pattern inreciprocal space. The periodic structure of a crystalline solid (whetherBCC or FCC) acts as a diffraction grating, scattering the electrons in apredictable manner. Working backwards from the observed diffractionpattern, the crystal lattice structure can be determined.

As used herein, the term “amorphous” means a non-crystalline solid.Amorphous materials can have small crystalline areas. In amorphousmaterials, at least 95% of the material is amorphous.

As used herein, the term “magnetic anisotropy” means the magnetizationprefers to orient in a particular direction.

As used herein, the terms “perpendicular magnetic anisotropy” and “PMA”mean the magnetization prefers to orient perpendicular to the xy-plane.PMA can be determined by measuring magnetic hysteresis loops in bothin-plane and out-of-plane directions.

As used herein, the terms “magnetoresistance” or “MR” refers to amagnetic tunnel junction's property of changing the value of itselectrical resistance in the presence of an external magnetic field. MRcan be measured by applying a magnetic field in one direction andmeasuring the resistance, and then applying a magnetic field in adifferent direction and measuring the resistance.

Spin torque MRAM has disadvantages that must be overcome before it canbe manufactured. First, the MR must be increased to enable smaller bitreading. For high MR with magnesium oxide (MgO) tunnel barriers, thefollowing features are optimal: 1) the material should have atwo-dimensional (2D) interface with MgO that has a square lattice net;2) the material should be a sufficient lattice match to MgO; and 3) thematerial at the interface should have a spin polarization of the Δ₁bands. Although BCC Fe-based alloys (e.g., CoFe or CoFeB) can be used,making BCC materials exhibit PMA is challenging.

Thin film magnetization generally lies in the plane of the film(in-plane magnetic anisotropy) in order to minimize the magnetostaticenergy. However, a PMA axis is necessary for efficient spin torqueswitching. Multilayer systems such as Co|Ni, Co|Pd, and Co|Pt havestrong PMA when they have sufficient face-centered cubic (FCC) (111)crystal orientation. However, the FCC structure does not providesufficiently high MR with MgO tunnel barriers that need BCC structure.It is challenging to develop magnetic materials systems thatsimultaneously provide high MR and large PMA.

The present disclosure solves the above problems by providing amultilayer magnetic material that has both high MR and large PMA. Themagnetic materials include multilayers of BCC materials and amorphousmaterials, in any combination. For example, the multilayer magneticmaterials include repeating BCC|BCC layers, BCC|amorphous layers,amorphous|BCC layers, or any combination thereof. The PMA arises fromthe anisotropy at each interface, or potentially from strain.

Referring to FIG. 1A, a cross-sectional view of a multilayer magneticmaterial 100 according to an exemplary embodiment is shown. Themultilayer magnetic material 100 includes repeating (or alternating)iron layers 110 and cobalt layers 120. Both the iron and cobalt layers110 and 120 are magnetic. Iron layer 110 includes iron and is BCC. Ironlayer 110 can include an iron alloy. Non-limiting examples of suitablematerials for the iron layer include iron, cobalt, boron, aluminum,nickel, silicon, oxygen, carbon or any combination thereof. Iron layer110 includes iron in an amount of at least 50 at. %. In someembodiments, iron is present in an amount of at least 50, 55, 60, 65,70, 75, 80, 85, 90, or 95 at. %. The multilayer magnetic material hasPMA.

Cobalt layer 120 includes cobalt and is either BCC or amorphous. Cobaltlayer 120 includes a BCC or amorphous cobalt material. The cobalt layer120 can include a cobalt alloy. Non-limiting examples of suitablematerials for the cobalt layer 120 include cobalt, zinc, beryllium,vanadium, boron, magnesium, aluminum, silicon, oxygen, carbon, or anycombination thereof. For example, non-limiting examples of BCC cobaltalloys include cobalt zinc, cobalt beryllium, and cobalt vanadium.Non-limiting examples of amorphous cobalt alloys include cobalt boron,cobalt magnesium, cobalt aluminum, cobalt silicon, cobalt beryllium, andcobalt carbon. Cobalt layer 120 includes cobalt in an amount of at least30 at. %. In some embodiments, cobalt is present in an amount of atleast 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95 at. %.

The iron and cobalt layers 110 and 120 are thin to maximize theinterface anisotropy. The thickness of each of the iron and cobaltlayers 110 and 120 is between about 2 and about 10 Ångstroms (Å). Thethickness of each of the iron and cobalt layers 110 and 120 can be thesame or different. Each iron and cobalt layer 110 and 120 has athickness about or in any range between about 2, 3, 4, 5, 6, 7, 8, 9,and 10 Å.

The multilayer material includes any number of alternating iron andcobalt layers 110 and 120. The multilayer material includes at leastthree layers, for example a cobalt layer 120 sandwiched between two ironlayers 110 (Fe|Co|Fe). The multilayer material includes at least 3layers. The multilayer magnetic material includes any number of repeatsof iron and cobalt layers 110 and 120, with a single repeat beingsignified by Fe|Co or Co|Fe. For example, 1.5 repeats are signified byFe|Co|Fe, and 3 repeats are signified by Fe|Co|Fe|Co|Fe|Co. Anywherefrom about 3 to 100 repeats are used.

FIG. 1B is a cross-sectional view the multilayer magnetic material 101of FIG. 1A with a tunnel barrier layer 130 on a first surface 104according to an exemplary embodiment. As shown, the tunnel barrier layer130 is in contact with the iron layer 110. However, the tunnel barrierlayer 130 can contact the cobalt layer 120 in other embodiments (notshown). When the multilayer magnetic material 101 is incorporated into aMTJ, it is advantageous to have the iron layer 110 contact the tunnelbarrier layer 130 because the polarization of the delta₁ bands arehigher for Fe. Either the cobalt layer 120 or the iron layer 110 cancontact the tunnel barrier layer 130.

The tunnel barrier layer 130 adds anisotropy to the multilayerstructure. The tunnel barrier layer 130 is a non-magnetic, insulatingmaterial. A non-limiting example of a suitable material for the tunnelbarrier layer 130 includes magnesium oxide (MgO). The tunnel barrierlayer 130 is formed by radiofrequency (RF) sputtering in someembodiments. Alternatively, the tunnel barrier layer 130 is formed byoxidation (e.g., natural or radical oxidation) of a magnesium (Mg)layer. After oxidation, the MgO layer may then be capped with a secondlayer of Mg. The second layer of Mg may have a thickness of about 5{acute over (Å)} or less in some embodiments. The tunnel barrier layer130 total thickness is not intended to be limited. The tunnel barrierlayer can have a thickness, for example, in a range of about 5 to about30 {acute over (Å)}. The MgO tunnel barrier can have a rock saltcrystalline structure.

FIG. 1C is a cross-sectional view of the multilayer magnetic material103 of FIG. 1A with a tunnel barrier layer 130 on a second surface 102according to an exemplary embodiment. FIG. 1D is a cross-sectional viewof the multilayer magnetic material 103 of FIG. 1A with a tunnel barrierlayer 130 on both the first and second surfaces 104 and 105 according toanother exemplary embodiment. The thickness of the tunnel barrier layer130 on first and second surfaces 104 and 105 can be the same ordifferent.

FIG. 2A is a cross-sectional view of the multilayer magnetic material201 of FIG. 1B with a dusting layer 210 disposed between the tunnelbarrier layer 130 and the multilayer structure portion 220 according toan exemplary embodiment. FIG. 2B is a cross-sectional view of themultilayer magnetic material 202 of FIG. 1C with a dusting layer 210disposed between the tunnel barrier layer 130 and the multilayerstructure portion 220 according to another exemplary embodiment. Thedusting layer 210 can include iron, cobalt, boron, or any combinationthereof. When the dusting layer 210 includes iron, iron is present in anamount of at least about 20 at. %.

The dusting layer 210 can be inserted between the multilayer structureportion 220 and the tunnel barrier layer 130 to further increase MR. Thedusting layer 210 may have smaller PMA but higher spin polarization thanthe multilayer structure portion 220. A suitable dusting layer 210 canbe Fe-rich CoFeB or CoFe, e.g., Co₄₀Fe₆₀B₂₀.

The thickness of the dusting layer 210 is not intended to be limited andcan be any suitable thickness. The dusting layer 210 can have athickness in a range from about 5 to about 20 Å thick. The dusting layer210 can have a thickness about or in any range from about 5, 10, 15, 20,or 25 Å.

Full MTJs can be formed by using two multilayer portions as describedabove, one on either side of the tunnel barrier layer. Dusting layersmay be used on both tunnel barrier layer interfaces. Alternatively, themultilayer portions may be used on only one side of the tunnel barrierlayer 130, and a different magnetic material can be used on the otherside.

FIG. 3A is a cross-sectional view of a MTJ 301 including the multilayermagnetic material as a free layer 330 and a reference layer 331according to an exemplary embodiment. Optionally, a dusting layer 210 isformed on top of the multilayer magnetic material forming the free layer330. The free layer 130 has a variable magnetization direction. A tunnelbarrier layer 130 is formed on top of the optional dusting layer 210.Alternatively, the tunnel barrier layer 130 is formed directly on themultilayer structure forming the free layer 330. Optionally, anotherdusting layer 210 is formed on the tunnel barrier layer 210. When morethan one dusting layer is used, the dusting layers can be the same ordifferent (e.g., different thicknesses and/or materials). The multilayermagnetic material is then disposed as a reference layer 331 on top ofthe dusting layer 210 or the tunnel barrier layer 130. The referencelayer 331 has an invariable magnetization direction. The reference layer331 can include the same number or a different number of iron layers 110and cobalt layers 120. The reference layer 331 also can includedifferent thicknesses of iron layers 110 and cobalt layers 120. The freelayer 330 and the reference layer 331 have PMA and are magneticallycoupled through the tunnel barrier layer 130.

FIG. 3B is a cross-sectional view of a MTJ 302 according to an exemplaryembodiment. As shown, the reference layer 331 is thicker than the freelayer 330. The multilayer magnetic material forms both the free layer330 and the reference layer 331. In other embodiments, the referencelayer 331 is has the same, similar, or thinner thickness compared to thefree layer 330 (not shown). Like in FIG. 3A, an optional dusting layer210 is formed on the multilayer magnetic material forming the free layer330. A tunnel barrier layer 130 is formed on the optional dusting layer210. Another optional dusting layer 210 is formed on the tunnel barrierlayer 130. The multilayer magnetic material forming the reference layer331 is formed on the dusting layer 210 or the tunnel barrier layer 130.The free layer 330 and the reference layer 331 have PMA.

FIG. 3C is a cross-sectional view of a MTJ 303 of FIG. 3B including aspacer layer 310 and a magnetic layer 320 according to an exemplaryembodiment. An optional dusting layer 210 is formed on the multilayermagnetic material forming the free layer 330. A tunnel barrier layer 130is formed on the optional dusting layer 210. Another optional dustinglayer 210 is formed on the tunnel barrier layer 130. The multilayermagnetic material forming the reference layer 331 is formed on theoptional dusting layer 210 or the tunnel barrier layer 130. The spacerlayer 310 is formed on the multilayer magnetic material forming thereference layer 331. The spacer layer 310 is any non-magnetic material.The magnetic layer 320 is formed on the spacer layer 320 and can includeany magnetic material. The free layer 330 and the reference layer 331have PMA The spacer layer 310 includes, for example, chromium,ruthenium, titanium nitride, titanium, vanadium, tantalum, tantalumnitride, aluminum, magnesium, oxides such as MgO, or any combinationthereof. The thickness of the spacer layer 310 is not intended to belimited. The spacer layer 310 can have a thickness, for example of lessthan 10, less than 5, less than 3, less than 2, or less than 1 Å.

The magnetic layer 320 includes a magnetic material. Non-limitingexamples of suitable magnetic materials include cobalt/platinummultilayers, cobalt/palladium multilayers, or any combination thereof.The thickness of the magnetic layer 320 is not intended to be limited.The magnetic layer can have a thickness of about 20-200 Å.

Any of the MTJs 301, 302, or 303 can be grown in any order. For example,the MTJs can be grown with the free layer 330 on top of the referencelayer 331 (not shown). The multilayer magnetic structure can be used asboth the free layer 330 and the reference layer 331 as shown in MTJs301, 302, and 303. The multilayer magnetic material also can be used asjust one of the free layer 330 or the reference layer 331.

It should be appreciated that the exemplary embodiments shown in FIGS.1A-D, 2A-B, and 3A-C and discussed above are shown for illustrativepurposes only. It is thus contemplated that other suitable multilayermagnetic materials and tunnel junctions may be formed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of making a magnetic material, themethod comprising: forming a cobalt layer on a first iron layer; andforming a second iron layer on the cobalt layer; wherein the first andsecond iron layers comprise iron and are BCC, the cobalt layer comprisescobalt and is BCC or amorphous, and the magnetic material has a PMA;wherein the magnetic material forms a portion of a spin torque magneticrandom access memory chip.
 2. The method of claim 1, further comprisingforming a tunnel barrier layer on the first or second iron layer.
 3. Themethod of claim 1, wherein the first iron layer comprises an iron alloy.4. The method of claim 1, wherein the cobalt layer is a cobalt alloy. 5.The method of claim 1, wherein the magnetic material forms a portion ofa magnetic tunnel junction.
 6. The method of claim 1, wherein themagnetic material forms a magnetic free layer of a magnetic tunneljunction.
 7. The method of claim 1, wherein the magnetic material formsa magnetic fixed layer of a magnetic tunnel junction.
 8. The method ofclaim 1, wherein iron is present in an amount of at least 50 atomic %(at. %) in the magnetic material.
 9. The method of claim 1, whereincobalt is present in an amount of at least 30 at. % in the magneticmaterial.
 10. The method of claim 1, further comprising forming amagnesium oxide layer on a surface of the magnetic material.
 11. Themethod of claim 1, further comprising forming a tunnel barrier layer onthe first iron layer or the second iron layer.
 12. The method of claim2, wherein the tunnel barrier layer comprises magnesium oxide.
 13. Themethod of claim 2, wherein the tunnel barrier is formed on an iron-richdusting layer.
 14. The method of claim 3, wherein the second iron layercomprises an iron alloy.
 15. The method of claim 6, wherein the magneticfree layer has a variable magnetization direction.
 16. The method ofclaim 7, wherein the magnetic fixed layer has an invariablemagnetization direction.
 17. The method of claim 10, wherein themagnesium oxide layer contacts the second iron layer.
 18. The method ofclaim 17, further comprising forming a dusting layer between themagnesium oxide layer and the second iron layer.
 19. The method of claim18, wherein the dusting layer comprises iron.